As reported previously, these investigations are primarily concerned with protein-protein interactions of the "limited proteolysis" type. The key reaction of blood coagulation is the conversion of fibrinogen to fibrin by thrombin. To this has been added the cross- linking of the fibrin. A newly instituted study of the thrombin catalyzed conversion of fibrinogen to fibrin and subsequent fibrin cross-linking in the lower species, salmon, has yielded the discovery of two new enzymes. One of these appears to rapidly destroy the fibrinogen or fibrin clots from salmon plasma and it is postulated that this enzyme may have some very practical applications. Lamprey plasma has been added to the lower species under investigation. Lamprey is the lowest of vertebrates, the first animal to possess a backbone. Although the most widely separated from man in time, certain aspects of blood coagulation appear to proceed along similar mechanisms. There is, however, an apparent molecular difference in the fibrinogen of man vs lamprey; yet, interestingly enough, the major blood clotting reactions studied bear a close similarity. But experiments of the past year are indicative that the alpha-chains of lamprey fibrinogen (fibrin) play only a minor role, if at all, in the formation of a stable blood clot. Of the 3 chains of lamprey fibrin, formulated alpha 2 Beta 2 gamma 2, the gamma cross-linking is sufficient to evoke stable clot formation.